Knife sharpener



FIG. 1 is a bottom, rear perspective view of a knife sharpener, showing my new design;

FIG. 2 is a top, front perspective view thereof;

FIG. 3 is a rear elevation view thereof;

FIG. 4 is a front elevation view thereof;

FIG. 5 is a bottom plan view thereof;

FIG. 6 is a top plan view thereof;

FIG. 7 is a right end elevation view thereof; and,

FIG. 8 is a left end elevation view thereof.

The broken lines shown in the drawings illustrate portions of the knife sharpener that form no part of the claimed design. 

CLAIM The ornamental design for a knife sharpener, as shown and described. 